This invention relates to fabricating workpieces, such as printed circuit boards, and in particular to providing an appropriate amount of solder at desired locations.
In a typical circuit board fabrication sequence, once the conductive features are formed, solder must be applied to the bonding pads to permit bonding of IC devices. One popular approach is to dip the board in molten solder and then remove excess solder by directing hot air at the board surfaces as they emerge from the solder bath. This technique is referred to as Hot Air Solder Leveling (HASL).
FIG. 1 illustrates a portion of a typical printed circuit board, 10, which includes a plurality of bonding pads, e.g., 11, for bonding thereto a semiconductor integrated circuit device (not shown). Conductive lines, e.g., 12, are also defined on the board in order to electrically couple the pads to other portions of the circuit. The circuit board also includes through-holes, e.g., 13, to provide electrical connection between conductors on the front and back surfaces of the board.
Most present HASL systems were designed for the old style circuit boards which utilized through-holes for attaching components rather than the solder mount pads (11) which are prevalent today. Consequently, present systems tend to leave a thin and/or uneven layer of solder on the pads which can result in costly defects in the final board. Merely reducing the pressure of the air is not generally effective since solder will not be cleared from the through-holes (13) which remain on the board.
It is, therefore, an object of the invention to provide printed circuit boards with the right amount of solder at surface mount and through-hole features.